Lithium-ion secondary batteries are common in portable consumer electronic devices because of their high energy-to-weight ratios, lack of memory effect, and slow self-discharge when not in use. Rechargeable lithium-ion batteries are also being designed and made for use in automotive applications to provide energy for electric motors to drive vehicle wheels.
Lithium-ion batteries may be formed in different sizes and shapes but three common functional components are the anode, cathode, and electrolyte that make up cells of the battery. Technically, the anode on discharge becomes the cathode on charge, and the cathode on discharge becomes the anode on charge. From here forward, we shall refer to the electrode that is the anode on discharge (the negative electrode) as the anode, and the electrode that is the cathode on discharge (the positive electrode) as the cathode. Typically, a porous separator is used to contain electrolyte and prevent physical contact (electron-conducting contact) between the anode and cathode. Many cells may be arranged in series or parallel electrical current flow connection, or any suitable combination thereof, to meet the electrical potential and power requirements of a battery design.
A lithium-ion battery generally operates by reversibly passing lithium ions between a negative electrode and a positive electrode. Typically, the negative and positive electrodes are situated on opposite sides of a microporous polymer separator that, along with the electrodes, is soaked with an electrolyte solution suitable for conducting lithium ions. Typically, each of the negative and positive electrodes is also carried on, or connected to, a metallic current collector (generally copper for the anode and aluminum for the cathode). During battery usage, the current collectors associated with the two electrodes are connected by a controllable and interruptible external circuit that allows an electron current to pass between the electrodes to electrically balance the related transport of lithium ions through each cell. Many different materials may be used to produce these various components of a lithium-ion battery. But in general, the negative electrode typically includes a lithium insertion material or alloy host material, the positive electrode typically includes a lithium-containing active material that can react with lithium at higher potential than the reaction with lithium at the negative electrode, and the electrolyte solution typically contains one or more lithium salts dissolved and ionized in a non-aqueous solvent. The contact of the anode and cathode materials with the electrolyte results in an electrical potential between the electrodes and, when an electron current is permitted to spontaneously flow during discharge in an external circuit between the electrodes, the potential is sustained by electrochemical reactions within the cells of the battery.
A lithium-ion cell or battery, or a plurality of lithium-ion batteries that are connected in series direct current flow or in parallel flow arrangement (or any suitable combination thereof) for current flow, can be utilized to reversibly supply power to an associated load device. The battery system delivers electrical power on demand to a load device such as an electrical motor until the lithium content of the negative electrode (anode) has been depleted to a predetermined level. The battery may then be re-charged by passing a suitable direct electrical current in the opposite direction between the electrodes.
At the beginning of the discharge, the negative electrode of a lithium-ion battery contains a high concentration of inserted lithium while the positive electrode is relatively depleted. The establishment of a closed external circuit between the negative and positive electrodes under such circumstances causes the transport of lithium from the anode to the cathode. The anode is spontaneously oxidized creating lithium ions and electrons. The lithium ions are carried through the micropores of the interposed polymer separator from the negative electrode (anode) to the positive electrode (cathode) through the ionically conductive electrolyte solution while, at the same time, the released electrons are transmitted through the external circuit from the negative electrode to the positive electrode (with the help of the current collectors) to balance the overall electrochemical cell by maintaining charge neutrality in the electrodes. The lithium ions spontaneously react with the cathode material by an electrochemical reduction reaction. The flow of electrons through the external circuit can power a load device until the level of intercalated lithium in the negative electrode falls below a workable level or the need for power ceases.
The lithium-ion battery may be recharged after a partial or full discharge of its available capacity. To charge or re-power the lithium-ion battery, an external power source is connected to the positive and the negative electrodes to drive the reverse of battery discharge electrochemical reactions. That is, during charging, the lithium within the positive electrode is oxidized to yield lithium cations and electrons. The cations transport across the separator to the negative electrode, and the electrons travel through the external circuit to the negative electrode as well. At the negative electrode, the lithium cations react with the negative electrode material through an electrochemical reduction reaction, and the negative electrode lithium content increases. Overall, the charging process reduces the lithium content within the positive and increases the lithium content within the negative.
In many lithium-ion battery applications it is preferred to periodically or continuously monitor the electrochemical potential of the electrode materials in the battery as a measure of their state of charge or state of health (their condition). Knowledge of the state of charge or state of health may be important for high charge rate or high discharge rate applications such as power tools and partially or fully electrified vehicles. The electrochemical potential of electrode materials may be altered and permanently lost if, for example, the cells of the battery are discharged too rapidly or overcharged often causing lithium to plate on the surface of the negative electrode. In order to monitor the electrode materials, a reference electrode has been placed in one or more cells of the battery in such a way as to monitor the state of charge of at least one or both of the positive and/or negative electrodes of the cell. The connection is a high impedance connection that draws very little current from the positive or negative electrode but the potential (voltage) of the positive and/or negative electrode in the cell electrolyte with respect to the reference is measured. These voltage values (reference electrode vs. positive and/or negative electrode) may be obtained in a battery as the cells are being charged or discharged, and collected for computer analysis and control of the discharge and charge rates of a battery.
However, the effectiveness of a reference electrode depends on its stability in an operating cell. There remains a need for improvement in the design and application of a reference electrode in a lithium-ion battery.